In gas turbine engines, fuel is burned within a combustion chamber to produce a hot effluent. The effluent is expanded within a turbine section across alternating rows of stationary stator vanes and rotating rotor blades, to produce useable power. Effluent temperatures at the initial rows of vanes and blades commonly exceed two thousand degrees (2000.degree. F.). Blades and vanes susceptible to damage by the hot effluent are cooled by air compressed upstream within the engine and bled to the turbine for cooling. Moreover, the turbine case is also susceptible to damage by the hot effluent.
Cooling of the turbine case is known in the art. Refer to U.S. Pat. No. 4,053,254 to Chaplin et al entitled "Turbine Case Cooling System." One substantial problem associated with cooling air used to cool the turbine case is the presence of particulates. The impingement of entrained particulates on the surface of the turbine case may erode the case. Inspection of turbine cases has shown that the case inner wall is susceptible to erosion corresponding to areas in close proximity to the cooling holes present in the turbine first vane support. Particulates pass through the cooling holes in close proximity to the turbine case and cause local areas of erosion in the case walls.
Erosion as considered here has to do with the removal of surface material by impingement of solid particulates. These particulates are present in the working medium flow path either as foreign matter ingested from the engine operating environment or as byproducts inherent in the combustion process. The majority of the particulates are very fine in terms of the size and are carried through the different sections of the engine. The erosion caused by the particulates entrained in the cooling air is dependent, at least in part, on the velocity of the particulates. Erosion of ductile materials such as copper, 6061 aluminum and annealed SAE 1215 steel, is proportional to the velocity raised to the power "a" (V.sup.a), where the exponent "a" is a value of 2.4-2.7. It has also been determined, with all other conditions being equal, that erosion wear upon steel depends on the diameter of impinging particulate matter. The larger the diameter of the particulates the greater the erosion wear. For turbine cases, the volume of material erosively removed is approximately proportional to the velocity of the particulates to the 2.5 power. Thus, as the velocity of the cooling airstream having entrained particulates increases, so does the erosion caused by the entrained particulates.
Inspection of prior art high pressure turbine case inner walls has revealed areas of local erosion of material, which appears as pitting in the case wall. The maximum allowable turbine case erosion depth according to industry standards is 0.010 inches. However, under actual engine operating conditions erosion depths on the order of 0.030 inches have been observed. Thus, the amount of erosion has in some cases progressed beyond this allowable limit. Local stress increase in the area of the pittings reduces the turbine case strength. This condition can occur in a very short period of time and cycles of engine operation if sand conditions are present such as would be the case in a desert environment.
There are several possible solutions to the erosion problem associated with turbine cases caused by particulates entrained in the cooling airstreams. The current approach for reducing the erosion damage is circumferentially adjusting the position of the high pressure turbine first vane support on the stator slightly, thus moving the cooling holes to a second location and allowing different areas of the turbine case to be eroded due to particulate impingement. However, this is a short-lived solution as only one circumferential adjustment is possible. Further adjustments would lead to positioning eroded areas on the turbine case to be close together which would eventually destroy the case. Another possible solution would be increasing the turbine case wall thickness. However, such addition of material would have a significant impact on turbine weight and invariably engine weight which is undesirable. In addition, the case albeit thicker, will still be eroded in areas subject to particulate impingement.